A single 500 mg dose of tinidazole was given to fourteen healthy volunteers. Cellular immune functions were studied, before and 2 and 24 h after drug ingestion. Blast transformation of lymphocytes to Con-A was significantly lower (P less than 0.01) in samples obtained 2 h after ingestion. No significant changes were found in the other immunological parameters studied. We conclude that these findings could be caused by reversible blockage of lectin binding receptors on the lymphocyte surface.